A-tom, i.e. in-dividual

How to count atoms?

LORENZO ROMANO AMEDEO CARLO AVOGADRO Caricature-donated by William Jensen, University of Cincinnati http://www.woodrow.org/teachers/chemistry/institutes/1992/AVOGADRO.GIF

The best way is on your fingers. But you need a very small finger!

1. Quite precisely atoms can be „counted” by X-rays: they are diffracted on crystal planes, like the light on a CD disk. Closer are placed the atoms – more distant are light spots on the screen.
2. There are others ways of counting atoms (i.e. calculation of the Avogadro or Loschmidt number). For example from the  sedimentation rate in liquid (J. Perrin) or from Brown’s motion (A. Einstein).
3. Today is possible to count on „fingers” leading it on the crystal surface. But the finger must be quite small and precise: the best is a sharp tip of the tungsten needle – when it touches an atom (or better: approaching it, then needle gets slightly attracted.

We call it Atomic Force Microsopy

Perspective color view of graphite surface http://www.physics.leidenuniv.nl/sections/cm/ip/group/Principle_of_SPM.htm	Carbon Monoxide Man on Platinum (111) http://www.almaden.ibm.com/vis/stm/images/stm5.jpg

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